The present invention relates to a high pressure discharge lamp, particularly of small dimensions, containing a getter device.
High pressure discharge lamps (also known as high intensity discharge lamps) are lamps in which the light emission is due to the electric discharge that is established in a gaseous medium comprising a noble gas (generally argon, with the possible addition of minor amounts of other noble gases), and vapors of different metals according to the kind of lamp.
These lamps are classified according to the means in which the discharge takes place. A first type are the sodium high pressure lamps, wherein the discharge means is a mixture of sodium and mercury vapors (obtained through vaporization of an amalgam of the two metals) and wherein, in operation, the vapors can reach pressures of about 105 Pascal (Pa) and temperatures higher than 800° C.; a second type are the mercury high pressure lamps (discharge in mercury vapors) wherein the vapors can reach pressures of about 106 Pa and temperatures of about 600-700° C.; finally, a third type of high pressure discharge lamps are metal halides lamps, wherein the discharge means is a plasma of atoms and/or ions created by the dissociation of sodium, thallium, indium, scandium or Rare Earths iodides (generally, each lamp contains at least two or more of these iodides), in addition to mercury vapors; in this case, with a lamp being turned on, pressures of 105 Pa can be reached in the burner and temperatures of about 700° C. in the coolest point of the lamp.
In FIG. 1 a generic high pressure discharge lamp, of the type wherein the electric connectors are on one side only of the lamp, is shown in a sectional view. Although in the rest of the description reference is always made to this type of lamp, the invention can be also applied in the so-called “double-ended lamps”, wherein there are electric contacts on both ends of the lamp. The lamp L is formed of an external bulb C, generally made of glass, inside which the so-called burner B is provided, formed of a generally spherical or cylindrical container of quartz or translucent alumina. Two electrodes E are present at two burner ends, and a noble gas added with a metal or a metal compound in vapor form (or vaporizable with the lamp turned on) V is provided inside thereof, the mixture of noble gas and the vapor being the means in which the discharge occurs. As known in the field, an end A of the bulb, and two ends Z of the burner are sealed by heat compression. The burner is kept in place by two supporting metal parts M, through metal feedthroughs R, these latter being fixed in parts Z by heat compression sealing these latter around the feedthroughs. The combination of the two parts M and R has also the function of electrically connecting the electrodes E to the contacts P external to the lamp. The space S enclosed in the bulb can be evacuated or filled with inert gases (normally nitrogen, argon or mixtures thereof). The bulb has the purpose of mechanically protecting the burner, thermally insulating this from the outside and, above all, keeping an optimal chemical environment outside the burner. Despite the provision of a particular atmosphere in the bulb, traces of impurities are always present in the lamp, for instance as a consequence of the manufacturing operations of the lamps, coming from outgassing or decomposition of components of the lamps or due to permeation from the external atmosphere. These impurities need to be removed, as they can alter the optimal lamp operation according to various mechanisms. Oxidizing gases possibly present outside the burner, due to the temperatures reached in the vicinity thereof, could damage the metal parts being present (parts M or R). Hydrogen, if present in the bulb, can easily permeate through the burner walls at the operating temperatures of these lamps, and once in the burner it has the effect of enhancing the potential difference between the electrodes E required for establishing and maintaining the discharge, thereby increasing the lamp power consumption. In addition, this rise of potential difference causes a rise in the electrodes “sputtering” phenomenon, consisting in the erosion thereof due to the impact of the ions present in the discharge, with consequent formation of dark metallic deposits on the burner internal walls and decrease of the lamp brightness. For these reasons, hydrogen is commonly considered the most noxious impurity in lamp bulbs.
To remove these impurities, it is known to insert in the bulb, outside the burner, a getter material capable of chemically fixing them. The getter materials are generally metals like titanium, zirconium, or alloys thereof with one or more transition elements, aluminum or Rare-Earths. Getter materials suitable for the use in lamps are described, for example, in U.S. Pat. Nos. 3,203,901 (zirconium-aluminum alloys); 4,306,887 (zirconium-iron alloys); and 5,961,750 (zirconium-cobalt-Rare Earths alloys). For the sorption of hydrogen, particularly at high temperatures, the use of yttrium or alloys thereof is also known, as described, for example, in British Patent GB 1,248,184 and in the International patent application publication WO 03/029502. Getter materials can be inserted in the lamps in the form of devices formed of the material only (for example, a sintered getter powder pellet), but more commonly these devices comprise a support or metallic container for the material. In FIG. 1 is shown a getter device C, typically used in lamps, formed of a thin metal plate on which a pellet of getter material powder is fixed. The drawing also shows a very common way of getter assembly to the internal structure of the lamp, in the so-called “flag” position. An example of a lamp containing a getter in the bulb is disclosed in International patent application publication WO 02/089174.
However, the known mountings of getter devices inside lamp bulbs have the drawback of causing a “shadow” effect, shielding the light coming from the burner for a solid angle depending on dimension of the getter device, its closeness to the burner, and its orientation with respect to the burner. This effect is undesired by lamp manufacturers, as it reduces by some percent units the overall lamp brightness. The shadow effect is a felt problem with conventional high pressure discharge lamps, which have relatively large dimensions (the bulb generally has a length greater than 10 cm). It becomes much worse in high pressure discharge lamps of recent development which have sensibly reduced dimensions, for example with bulbs having an external diameter of about 2 cm or less and length of less than 7 cm (in the remaining part of the text, high pressure discharge lamps with these dimensions will be referred to as miniaturized lamps). With such reduced dimensions, positioning the getter device inside the bulb presents a number of problems. In the first place, there is a direct effect: a bulb of reduced dimensions forces positioning the getter device closer to the burner compared to larger dimension lamps, so that, with the same dimensions of the getter device, the shadow effect is increased. In the second place, there is an indirect effect linked to the fact that the sorption of hydrogen by getter materials is (contrary to all other common impurities) an equilibrium phenomenon: the higher the temperature, the higher the pressure of gaseous hydrogen in equilibrium with the getter. With miniaturized lamps, any bulb location is at relatively high temperature and as a consequence, in order to guarantee sufficiently low pressures of gaseous hydrogen in the bulb, it would be necessary to increase the amount of getter material and thus the dimensions of the getter device. This increase in dimensions and the above mentioned need to place the device close to the burner concur to increase the shadow projected by the getter device.